Is it normal to have a bubble on the bottom of my board?

What about clusters of little bubbles?

The short answer is-- Yes to both. 

a larger bubble (about 3" in diameter) may occur directly under the board's valve. This is because there cannot be drop stitching in that area. The bubble will not spread or affect the performance of the board. It is simply cosmetic and you will never notice it as you paddle.

Your board may also have groups of tiny bubbles. This is completely normal! The layers of PVC are glued together and will commonly have small areas where dust or other small particles have settled during the manufacturing process. They too are cosmetic, and will not grow or affect the performance of your SUP board.